1. Field of the Invention
The present invention relates to an apparatus for and a method of processing images, and a recording medium. More particularly, the present invention relates to an apparatus for and a method of processing images, and a recording medium with which images subjected to the MPEG (Moving Picture Experts Group) coding in the form of open GOP (Group Of Pictures) can be reproduced without changing image quality.
2. Description of the Related Art
In the MPEG coding, an image of a certain frame (field) is predicted from frames before and behind the frame so that random access can be made in units of GOP comprising several frames grouped together.
Meanwhile, the GOP is divided into a closed GOP in which prediction is performed by using only images belonging to itself, as shown in FIG. 1A, and an open GOP in which the first B-picture of a certain GOP is predicted by using not only an I-picture belonging to that GOP, but also a P-picture forming one image of the preceding GOP, as shown in FIG. 1B.
On condition that a quantity of codes is kept the same, the closed GOP is inferior in image quality to the open GOP. Conversely, to obtain image quality by the closed GOP at the same level as obtained by the open GOP, a quantity of codes is increased. From the standpoints of improving image quality and reducing a quantity of codes, therefore, the open GOP is generally employed.
The open GOP has however accompanied a problem below. When image editing of, for example, inserting in one MPEG bit stream X shown in FIG. 2(A) the other MPEG bit stream Y shown in FIG. 2(B) is performed, image quality is remarkably deteriorated in hatched portions of the decoded images, shown in FIG. 2(c), at the joint between the bit streams X and Y.
More specifically, of B-pictures of images forming one GOP, those B-pictures locating upstream of an I-picture in the same GOP (hereinafter referred to as before-I-picture B-pictures) are predicted in two-way directions from the I-picture and a P-picture as one of images forming the preceding GOP. When the other bit stream Y is joined to one bit stream X, therefore, each of the before-I-picture B-pictures in the first GOP of the bit stream Y is predicted by both backward prediction using an I-picture in that GOP (current GOP) and forward prediction using a P-picture in the last GOP (preceding GOP) of the bit stream X which has no relation with the bit stream X, as shown in FIG. 3. This has raised a problem that block noises are generated and image quality is remarkably deteriorated.
To overcome such a problem, there is proposed a method of setting a broken link flag specified in the GOP layer, for example, in the bit stream after being edited, and causing an MPEG decoder to recognize that the bit stream has been edited. Then, the MPEG decoder does not display the before-I-picture B-pictures in the GOP for which the broken link flag is set, but continues to display the image prior to those before-I-picture B-pictures.
The MPEG decoder is however not always designed to be able to detect the broken link flag and hence cannot deal with all the cases.